The present invention relates, in general, to electrical watthour meters and, specifically, to electrical watthour meter sockets, and more specifically, to watthour meter sockets and bypass devices for use with current and/or potential transformers.
In the electric utility industry, watthour meters are employed to measure electrical power consumption at a residential or commercial building establishment. A cabinet is typically mounted on an inside or outside wall of the residence or building use site and contains a meter socket having line and load contacts which are connected to electric utility power line conductors and electric load conductors connected to the residential or building site power distribution network. The contacts receive blade terminals of a plug-in watthour meter to complete an electric circuit through the meter between the line and load terminals mounted in the cabinet for the measurement of electrical power consumption.
The meter socket is typically mounted on the rear wall of the cabinet by fasteners, such as bolts, which extend through the meter socket into the rear wall. Alternately, the meter socket can be mounted on a separate back panel over threaded studs mounted on the rear wall of the cabinet and extending inward through apertures in the panel to receive nuts.
Current transformer or CT rated watthour meters and socket adapters are employed in high current applications. The current transformers coupled to the line and load conductors have their output leads connected to terminals in a current transformer or CT rated watthour meter socket. A low current rated watthour meter is then plugged into the socket to measure power consumed at the use site.
In addition, potential coils in a watthour meter may also be connected via potential blade terminals to potential jaw contacts mounted in the socket and connected by individual conductors to terminals mounted in the terminal portion of the socket.
However, with current transformer rated socket adapters or sockets, it is necessary to short circuit the line and load terminals when the watthour meter is removed from the socket for replacement or testing. Heretofore, bypass devices, such as test switches or slidable link test blocks have been used with CT rated sockets to provide the necessary short circuit or bypass feature.
Exemplary test switches and link test blocks are made by Meter Devices Company, Inc. of Canton, Ohio. Such bypass devices are typically mounted in a watthour meter socket. The test switches are generally in the form of single throw, knife-type switches which are provided in multiples ganged together into one assembly; but each switch is electrically connected between one line contact or one load contact in the socket and one current transformer lead. Once the socket cover is removed, the test switches can be operated as desired to provide the necessary bypass connection between the line and load contacts and the conductors from the current transformers prior to removing the watthour meter from the socket for testing, recalibration, replacement, etc., and the reinstalling a watthour meter in the socket.
In use, the meter socket itself or the meter socket and a bypass device are mounted in a socket enclosure or housing. The individual socket terminals are then wired to the bypass device terminals.
It is also known to mount a meter socket, such as an Ekstrom Industries, Inc., 2100 Series, bottom feed to socket-type adapter, on a mounting panel along with a bypass test switch assembly. The meter socket and the test switch assembly are then mounted by fasteners on a meter enclosure back panel or in a meter socket housing
Ekstrom Industries, Inc. has sold a 2100 adapter with built-in or integrally mounted test switches which are mounted below the socket adapter jaw contacts in place of the normal power terminals as shown in U.S. Pat. Nos. 6,475,028 and 6,488,535.
440/480 volt single or three-phase service is typically provided to large electric power users. Due to the high current requirements of such high power users, current transformers are typically coupled to the utility power conductors. However, the 440/480 voltage provided at the current transformers remains unchanged by the current transformers thereby requiring an expensive 480 volt-rated watthour meter for power measurement.
However, 480 volt-rated equipment is more volatile and more susceptible to failure, such as arc faults which can lead to phase-to-phase or phase-to-neutral faults and potentially cause a full flashover event.
In addition, high transient voltage spikes, due to lightening strikes, line surges, fault currents, etc., could cause voltages that exceed the meter or meter socket capacity. Old-style watthour meters incorporated surge gaps to provide over-voltage to ground protection, such as from a lightening strike. Such surge gaps provide protection for over 1,000 volts for short periods. However, existing electrical power standards do not require watthour meters to have surge gaps and thus most watthour meter manufacturers incorporate metal oxide varistors (MOV) in their meters.
This poses a problem since the MOVs do not provide much protection over 1,000 volts. In addition, ferroresonance which may be present when a watthour meter is plugged into the socket base could cause voltage spikes greater than 1,000 volts leading to meter failure and an explosive event within the meter socket posing hazards to any utility service personnel accessing the socket.
At the same time, it still remains extremely dangerous for utility personnel to work on 480 volt equipment when live voltage is present on such equipment. Such high voltage work requires the use of personal protection equipment in the form of full body suits, masks, etc.
One prior attempt to address these problems associated with working with live 480V equipment involved the use of 480V/120V potential transformers mounted in the socket next to the meter socket base and/or optional meter test switches. The potential transformers transform the high 480 volt to lower 120 volt service. This allowed the use of less expensive and more easily handled 120 volt rated watthour meters.
Another prior attempt to address these concerns involved the mounting of a VT-pack voltage transformer assembly manufactured by Two Sockets-Two Meters, Inc., of South Dakota. The VT-pack is in the form of a small housing carrying three voltage transformers which transform the 480 volt service to 120 volt service or output conductors coupled to the voltage transformers. The output conductors are connected to a plug-in connector which receives a mating connector plug coupled at one end of a wiring harness which is pre-wired to the meter socket jaw contacts and/or optional meter test switches.
However, these designs still place the 480 volt rated equipment in the same cabinet or enclosure with the watthour meter equipment thereby necessitating the use of full personal protective equipment by the utility service person when installing, removing or otherwise servicing the watthour meter and/or optional meter test switches in the socket cabinet.